More species means less disease

Conserving biodiversity while reducing contact with humans can limit the spread of pathogens

Natasha Gilbert

In one study, the number of virus-carrying deer mice rose dramatically as the diversity of mammal species declined.Visuals Unlimited/naturepl.com

Biodiversity protects ecosystems against infectious diseases, researchers have concluded. The finding suggests that loss of species from an environment could have dangerous consequences for the spread and incidence of infections, including those that affect humans.

Felicia Keesing, a biologist at Bard College in Annandale, New York, and her colleagues reviewed several dozen studies published in the past five years and found that the link holds true across various ecosystems, pathogens and hosts. "A pattern is emerging which shows that biodiversity loss increases disease transmission," says Keesing, whose study is published today in Nature1.

The researchers don't know why the effect occurs. But they speculate that species that are better at buffering disease transmission — for example because they have low rates of reproduction or invest heavily in immunity — tend to die out first when diversity declines, whereas species that have high rates of reproduction or invest less in immunity — and thus are more likely to be disease hosts — survive for longer.

The review analyses studies of 12 diseases, including West Nile fever and Lyme disease, in ecosystems around the world. In every study, the diseases became more prevalent as biodiversity was lost. For example, three studies showed that a decreased diversity of small mammals in an area causes the prevalence of hantaviruses — which induce fatal lung infections in humans — in host animals to rise, thereby increasing the risk to humans.

“The clear message is that we degrade ecosystems at our own peril.”

Will Turner Conservation International

One of the three studies, based in Oregon, found that the prevalence of the Sin Nombre hantavirus in deer mouse (Peromyscus) populations increased from 2% to 14% as the diversity of mammal species declined in the area. A study in Utah found similar results. In the third study, researchers experimentally reduced the diversity of small mammals on several study plots in Panama. The number of animals that were hosts to the virus increased from around five per plot to more than six.

In other examples, three separate investigations found strong links between low bird diversity and increased incidence of West Nile encephalitis in the United States. Communities with low bird diversity were dominated by species susceptible to the virus; this induced high infection rates in mosquitoes and people. By contrast, communities that were home to a greater range of birds contained many species that were not good hosts for the virus.

Direct contact

But when the researchers looked at how biodiversity affects the emergence of new pathogens, they found mixed results. One study concluded that the probability of pathogens jumping from wildlife to humans is actually higher in areas that are rich in biodiversity.

"Biodiversity could be a source of new diseases, but once a disease emerges, greater biodiversity is protective," says Keesing.

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Keesing and her team reanalysed the data in this disease-emergence study and found that almost half of the new diseases were connected with changes in human land use and agricultural and food production practices — including hunting bushmeat — all of which increase contact between people and wildlife. The team suggests that this greater interaction, rather than the rich biodiversity, could have caused the increased disease emergence.

"Preserving large intact areas and minimizing contact with wildlife would go a big step of the way to reducing disease," says Keesing.

"The review makes a strong case that biodiversity can help stop the spread of infectious diseases," says Will Turner, an ecologist and director of conservation priorities at Conservation International, a campaign group based in Arlington, Virginia.

There is a long way to go before researchers can understand the mechanisms at work, but "the clear message is that we degrade ecosystems at our own peril", says Turner.

I once saw a documentary which showed that India has widespread yellow fever virus but no vectors (mosquitoes I think), my question is with the rate of air/sea travel between the countries what are the chances a few vectors would make their way to India?